The freshwater calanoid copepod Diaptomus sanguineus switches each year in spring from making eggs that hatch immediately to making diapausing eggs that rest in lake sediments for an extended period. In lakes and ponds containing planktivorous fish, the timing of the switch is highly consistent between years and comes at the theoretically optimal time (late March) to avoid intense summer predation. In fishless ponds the timing comes 1-2 mo later. We investigate here the environmental cues used by the copepods to time the switch to diapause. Through the use of both field and laboratory manipulations, we show that temperature and photoperiod play central roles as diapause stimuli. A field manipulation of fish density failed to reveal either a direct induction of diapause or any more subtle effects of fish occurrence on diapause timing. The copepods made immediately hatching eggs under short-day or low-temperature conditions, and diapausing eggs under long days or high temperatures. There appears to be variation between individuals in their responses to temperature and photoperiod so that similar diapause phenologies are produced by different sensitivities to the environmental cues.